The present invention relates to nonlinear optical materials having a high nonlinear optical constant useful for doubling the frequency of a laser beam and the like.
Nonlinear optical materials have actively been investigated as the important technology for optoelectronics in such applications as laser beam frequency doublers, optical modulators, optical switches and optical computers. Well known nonlinear optical materials include lithium niobate, potassium dihydrogen phosphate and the like. Unlike these inorganic materials, no crystals of organic materials have been developed which have a macroscopic second order hyperpolarizability large enough for practical use, although the ones available possess superior characteristics such as a nonlinear optical constant 100-1000 times higher, response at a higher speed and stronger resistance to optical damages than the inorganic materials. In many cases, even when the constructing molecule itself has a large secondary molecular susceptibility, the macroscopic second order hyperpolarizability will be zero in the crystalline state due to its inversion symmetry with the result that there will be no optical secondary harmonic generation (SHG). As stated above, it is difficult to produce single crystals with no center of symmetry by the use of a low-molecular organic substance alone. Therefore, methods have been proposed in which such organic molecules are dispersed in a matrix of a macromolecular compound and the host macromolecules are oriented by the aid of an external field, such as an electric field, simultaneously allowing for unsymmetric orientation of the guest organic molecules. For example, SHG is observed by blending 4-dimethylamino-4'-nitrostilbene (DANS) in a nematic liquid crystalline macromolecules and then applying an electric field to the blend to cause orientation [G. R. Meredith, Macromolecules, 15(5), 1385(1982)]. In this method, however, as the DANS can be blended in a concentration of 2% maximum and the orientating force of the host macromolecular liquid crystals is not satisfactorily high, there is produced an unsatisfactory nonlinear optical constant several times as high as that of urea. Japanese Patent Laid-open No. 238538/1987 also describes an observation of an SHG of 0.53 .mu.m by incidence of Nd.sup.3+ /YAG laser beam upon a film prepared from a composition of DANS dispersed in thermotropic macromolecular liquid crystals of main chain type. Also, EP No. 244288 describes the synthesis of a liquid crystalline polymer of side-chain type in which a mesogen and a unit exerting nonlinear optical response have been linked to the side chain of a polymer, such as polyacrylate, polymethacrylate or polysiloxane, and the orientation of a film of the polymer by applying an electric field followed by incidence of Nd.sup.3+ /YAG laser beam by which an SHG of 0.53 .mu.m is observed. The methods described in these publications are commonly characterized by the use of a matrix polymer which is the nematic liquid crystalline polymer of a substance inherently having a symmetrical center since a unit exerting a nonlinear response is orientated asymmetrically and consequently has a critical disadvantage in that a material having a nonlinear optical constant sufficiently high for practical use cannot be produced by these methods.
Another disadvantage of the method of using a nematic liquid crystalline polymer is that the use of a nematic liquid crystalline polymer produces no benefit in the phase matching which is technologically important for practical use, and utilization of conventional birefrigence or optical waveguide is necessary.